The present invention relates to a method for decreasing problems caused by the presence of residual and/or reaction gases entrained in a process liquid and to apparatus for carrying out such method. More particularly, the present method and apparatus especially relate to processes in the manufacture of pulp and paper.
A pulp or paper process is a sum of several successive sub-processes and reactions and as these progress, the amount of gases entrained in the liquid medium will increase and decrease. Often, the presence of such gases is undesirable to other sub-processes and to the final product resulting therefrom as well as to the environment. According to the traditional technology, residual and reaction gases are conveyed with the rest of the material out of a process into a degassing plant, where they are either released to the atmosphere or conveyed further into an effluent treatment.
Gas is present in pulp suspensions mainly in three forms, namely, in the form of small bubbles, dissolved or chemical bound gas.
The chemically bound gas or dissolved gas seldom causes problems in the pulp and papermaking processes but can cause problems if conditions are changed and bubbles start to form.
Gas bubbles in the fiber suspension can be present as free bubbles in the liquid between the fibers or as bound bubbles attached to fibers. Both bound and free bubbles cause problems in the papermaking processes. Free bubbles cause special problems in the pulp and papermaking processes when they are present in too great an amount. The problems include foam problems, instability of the processes, decreased deaerating, and the like.
Both free and combined gases impede pumping of the liquid in which such gas is entrained and, among other things cause pump cavitation, and, as a result of inadequate degassing, when gas gets into a pulp or paper process it may cause the following disadvantages:
free gas: foam problems; pinhole perforation of paper; instability in pipe conduits connected to screens, valves, etc.; decreased dewatering.
combined gas: dewatering problems; fiber flocculation; increased paper capillarity; decreased formability on a wire.
dissolved gas: if dissolved gas transforms to combined gas, it will exhibit the same problems as combined gas.
The method of the present invention relates to the separation and removal of most of the free air bubbles so that the problems caused by an excess amount of free air bubbles is eliminated.
Total gas removal is generally accomplished by another type of gas removal, so-called mechanical gas separation. With this method, all of the free and bound gas bubbles are removed. Also part of the dissolved gas is removed. This type of gas removal is performed immediately in front of the paper machine forming section to avoid pinholes and other problems on the forming wire. This method, which is described by K. D. Kurz, Tappi Engineering Conference, Sep. 19-21, (1978), is expensive and creates large amounts of foam when the fiber suspension is ejected with high speed onto a metal surface in a vacuum tank.
The traditional degassing assemblies in the pulp and paper industry are remarkably space demanding and hence costly, and the separated gas occurs in large volumes, from which reclaiming and conveying thereof is difficult. The most usual degassing equipment is a tank having a large diameter in which the gas in a gas contained liquid is permitted to rise to the liquid surface of the tank for removal. In order to be certain that a sufficient time period for degassing is given, the diameter of such a tank in large pulp plants can be 10-20 m and the height 5-6 m. It will thus be apparent to persons of ordinary skill that investment costs for a degassing tank of this kind are high and the reclaiming of gas therefrom is difficult.